Power semiconductor elements having large outputs but generating large amounts of heat, such as insulated gate bipolar transistors (hereinafter, referred to as IGBTs) need to be cooled in order to secure the reliability and longevity of their functions. In recent years, use of such power semiconductor elements has been expanding as they have been used in control of motors of electric automobiles and so forth. Hence, in addition to an improvement in output, improvements in reliability, longevity and the like are being demanded.    Patent Document 1: Japanese Patent Application Publication No. Hei 6-188363    Patent Document 2: the Pamphlet of International Patent Application Publication No. 98/43301